The present invention relates to an improvement in a multibeam array antenna for use in a radar, and more particularly to a multibeam array antenna based on a matrix 5 feed network system.
In radars, e.g. three dimentional radars which need precise information indicative of distance, azimuth and height or altitude in respect of a target, particularly, the accuracy of the azimuth and the height depends in large part upon antenna characteristics. For such radar antennas, pencilbeam array antennas having sharp directivity are suitable and there has been widely employed a system of scanning a predetermined space with the pencilbeam antenna at a high speed. However, such a scanning system using a single beam essentially requires an appreciable time for scanning the predetermined space, resulting in a restriction on the data updating rate for the target information, which is one of the important performance characteristics for radars.
To eliminate this restriction, a multibeam antenna system which concurrently forms a plurality of beams with the same antenna has been proposed. As one of methods of forming a beam suitable for the multibeam antenna system, the matrix feed network system is well known. This system is, for example, described in "Antenna Engineering Handbook" edited by Electro Communication Society and published by Ohm-Sha P 223, and "Microwave Scanning Antennas" edited by R.C. Hansen and published by Academic Press (1966) VOL. III, PP. 247-258 etc., and will be described in detail with reference to FIGS. 1 to 3.
Referring to FIG. 1, there is shown an example of eight-element, two-multibeam array antenna based on the prior art matrix feed network system. This array antenna designated by reference numeral 3 is configured in a matrix manner, which comprises multibeam ports 11 and 12 for input powers, a first series power feedline including power feedlines 21 and 22 connected, at one end, to the multibeam ports 11 and 12 and directional couplers 31a to 31h and 32a to 32h, radiation elements 5a to 5h for forming a beam, a second series power feedline including power feedlines 4a to 4h for mutually coupling the directional couplers 31a to 31h and 32a to 32h associated with the first series power feedline and the radiation elements, and resistive terminations 6a to 6h coupled to the power feedlines 4a to 4h and resistive terminations 6i and 6j coupled to the other end of the respective power feedlines 21 and 22.
FIG. 2 generally depicts beams formed by the array antenna shown in FIG. 1.
The basic operation of the above-mentioned antenna will be described with reference to FIGS. 1 and 2. Input power to the beam port 11 is successively distributed to the radiation elements 5a to 5h by the directional couplers 31a to 31h provided on the first series power feedline 21, thus forming a beam 1 shown in FIG. 2. Likewise, input power to the beam port 12 is also successively distributed to the radiation elements 5a to 5h by directional couplers 32a to 32h provided on the second series power feedline 22, thus forming a beam 2 shown in FIG. 2.
As far as the flow of the input power applied to the beam port 12 within the power feeding circuit is concerned, the following operation must be taken into consideration in addition to the above-mentioned basic operaiton.
Namely, the input power to the beam port 12 is normally transmitted to the second series power feedlines 4a to 4h by the directional couplers 32a to 32h thereby to excite the radiation elements 5a to 5h. However, in this power transmission, the input power partially leaks to the first series power feedline 21 through the directional couplers 31a to 31h to excite the radiation elements 5a to 5h through the directional couplers 31a to 31h provided on the first series feedline 21. The leakage power is radiated in the beam direction determined in principle by the first power feedline 21, i.e. in the direction of the beam 1. Accordingly, such a radiating beam due to the leakage power serves as a spurious lobe with respect to the beam 2 formed by exciting the beam port 12.
For instance, when the degrees of coupling of all the directional couplers are equal to each other, the level difference (L.sub.s) between the spurious lobe and the main beam is approximately expressed by the following equation in accordance with the above-mentioned reference "Microwave Scanning Antennas" P 254, EQU L.sub.s (dB).congruent.20 log.sub.10 (4 .pi.b/E) (1)
where b is the beam interval or the beam separation angle in beam width between the radiation beams 1 and 2 normalized by the half power width, and E is the efficiency of the feed network. For instance, if the beam interval b is set to the value equal to the half-power width (b=1) and the efficiency of the feed network is 75% (E=0.75), the level difference is expressed as L.sub.s =24.5 dB in acccordance with the above-mentioned equation (1). Namely, as shown in FIG. 3, with resepct to the main lobe of the beam 2, the spurious lobe 2a is generated in the direction of the beam 1, and has the level of -24.5 dB with resepct to the level of the main beam. In FIG. 3, the ordinate and the abscissa denote relative power and beam angle, respectively.
In general, the radar antennas are required to have low sidelobe and high efficiency. Accordingly, it is necessary to enlarge the beam separation angle b in order to meet this requirement in accordance with the relationship expressed by the equation (1). However, if the beam separation angle b is enlarged, the gain of the antenna at an angular crossover point of both the beams, i.e. a crossover level, will necessarily be lowered. As a result, there arises a problem that a necessary region for a radar system cannot be formed at this angular direction.
For this reason, the multibeam antenna based on the prior art matrix feed network system is disadvantageous in that there exists a restrictive relationship between the level of the spurious lobe and the crossover level between adjacent beams.